In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3GPP). The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected to a core network (via Access Gateways, or AGWs) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
In a cellular system a wireless terminal located in one cell may also receive interference from transmissions occurring in other cells, e.g., adjacent cells. Such Multi-cell interference in a cellular system is one of the most dominant sources of impairment. One solution for mitigating multi-cell interference is sometimes referred to as “Coordinated Scheduling”. In Coordinated Scheduling multiple cells are connected to a central unit that coordinates the transmission/reception to and from the wireless terminals so that interference can be avoided. The interference avoiding coordination of Coordinated Scheduling typically involves scheduling users who are compatible to be active simultaneously over the same radio resource.
Since the network architecture with a centralized control unit may not be feasible for various considerations, a distributed method for coordinated scheduling has recently been disclosed in U.S. patent application Ser. No. 12/486,202, entitled “Network-Wide Inter-Cell Interference Minimization Via Coordinated Multipoint Cell Scheduling Coordination”, which is incorporated herein by reference in its entirety. In this distributed method, the system is partitioned into sets of cells by a certain reuse factor. The cells in the same set do not interfere with each other due to the reuse distance and can therefore schedule data transmissions independently. The cells in different sets then communicate with each other to exchange scheduling information regarding anticipated data transmissions so that interference can be avoided during the data transmissions. This distributed method may approximate the performance of that of the centralized architecture, but does involve some complications.
In the above regard, the aforementioned distributed method assumes that the distributed coordination (e.g., scheduling information regarding the scheduled data transmissions) is completed before the actual data transmission begins. Operation premised on this assumption can pose a problem. To understand the problem, it must be realized that, in practice, the exchange of scheduling information takes place over an interface, e.g., an interface that connects two nodes or units that exchange the scheduling information. The interface has latency, and if the latency is sufficiently great the latency may negatively affect the responsiveness and performance of coordinated scheduling if the coordination procedure is not properly designed. In other words, the interface may have characteristics or encounter situations such that the exchange of scheduling information over the interface is slowed or delayed and thus a hindrance to the coordination and data transmission operations. An example of such an interface with the potential of such a problem may be an interface in a switched network (such as an X-2 interface in the case of LTE). Thus, the latency of the interface may frustrate the timely exchange of scheduling information and the subsequent data transmissions that are scheduled in accordance with the scheduling information.
The complications of the foregoing distributed coordination method may be understood with reference to FIG. 1. For simplification FIG. 1 shows a cell layout with a reuse factor of three. In the distributed coordinated scheduling method mentioned previously, the cells are divided into three sets, i.e., numbered sets 0, 1, and 2. In FIG. 1 each set of cells has a different interior design fill. For example, set 0 has a vertically hatched interior design; set 1 has a cross-hatched interior design; set 2 has a stippled or dotted interior design. The cells in a same set can schedule users independently since the cells of the same set are separated by the reuse distance (the reuse distance is assumed to be large enough so that inter-cell interference of same set cells is negligible). The cells in different sets, on the other hand, do interfere with their immediate neighboring cells and therefore should coordinate with each. One manner of coordination is described below.
The cells in each set take turn scheduling in a certain order (e.g., vertical interior fill, then cross-hatched interior fill, then stippled interior fill, for example), independently from other cells in the same set. The cells in the scheduling set should avoid causing interference to cells in the sets that have already scheduled, and then pass sufficient information to neighboring cells in the sets that are yet to schedule so that the same interference avoidance measure can be taken.
For example, following the order of set 0, set 1, and set 2, the cells in set 0 first schedule their users and pass that information to neighboring cells in set 1 and set 2. The cells in set 1 then schedule their users, making sure that the interference to users in neighboring cells of set 0 does not exceed a certain target, and then pass that information to neighboring cells in set 2. Finally, upon receiving the scheduling information from the neighboring set 0 and set 1 cells, the cells of set 2 proceed to schedule their users, making sure that interference to scheduled users in the neighboring cells of set 0 and set 1 does not exceed a certain target. All the scheduling and information passing should take place before the data transmission phase. The order in which the three sets perform their scheduling may be changed from time to time so that fairness can be maintained.
The assumption that the coordination process is completed before the data transmission takes place implies that a Coordination Time Interval (CTI) can be no greater than a Scheduling Time Interval (STI), as shown in FIG. 2. FIG. 2 particularly shows two essentially parallel tracks of processing: the foreground processing of user data transmissions which is comprised of Scheduling Time Intervals (STI), and background processing which includes the exchange of scheduling information in plural Broadcast Time Intervals (BTIs). If Broadcast Time Interval (BTI) is defined as the time it takes for a cell to communicate its scheduling information to the neighboring cells plus any other processing delay, then the Coordination Time Interval (CTI) must be no less than the Broadcast Time Interval (BTI) multiplied by the reuse factor. As illustrated in FIG. 2, before a Scheduling Time Interval (STI) can begin its corresponding background processing, which occurs in the corresponding Coordination Time Interval (CTI), must be completed.
In the case of Long Term Evolution (LTE), Broadcast Time Interval (BTI) can be loosely interpreted as the X2 interface latency, which ranges from a few msec to several hundred msec, depending on the deployment. For a reuse factor of 3, 4 or 7, the existing solution may have a Scheduling Time Interval (STI) on the order of a second.
A delay of this magnitude can significantly degrade the system performance due to factors such as mismatch in signal strength between the time of measurement and the time of data transmission. The achievable user data rate determined by link adaptation mechanism during the coordination period may also become invalid during data transmission, resulting in unnecessary retransmissions.
Furthermore, the distributed coordinated scheduling method disclosed previously did not consider the problem of link adaptation. Link adaption is essentially defined as the matching of modulation, coding and other signal and protocol parameters to the conditions of the radio link. The scheduling information is passed in one direction from the cells with higher order to those with lower order. To be able to perform adequate link adaptation, the scheduling information from all neighboring cells need to be available prior to the data transmission. The broadcast of scheduling information should therefore be in the opposite direction as well.
U.S. patent application Ser. No. 11/681,302, “METHOD AND APPARATUS FOR RESOURCE REUSE IN A COMMUNICATION SYSTEM,” filed Mar. 2, 2007, published as US 2008/0212539, and incorporated herein by reference, discloses a method for exchanging scheduling information in advance of data transmission so that more accurate link adaptation can be achieved.